base- Basic libraries

Copyright(c) The University of Glasgow, 2008-2009
Licensesee libraries/base/LICENSE
Safe HaskellTrustworthy



Types for text encoding/decoding



data BufferCodec from to state Source




encode :: CodeBuffer from to

The encode function translates elements of the buffer from to the buffer to. It should translate as many elements as possible given the sizes of the buffers, including translating zero elements if there is either not enough room in to, or from does not contain a complete multibyte sequence.

If multiple CodingProgress returns are possible, OutputUnderflow must be preferred to InvalidSequence. This allows GHC's IO library to assume that if we observe InvalidSequence there is at least a single element available in the output buffer.

The fact that as many elements as possible are translated is used by the IO library in order to report translation errors at the point they actually occur, rather than when the buffer is translated.

recover :: Buffer from -> Buffer to -> IO (Buffer from, Buffer to)

The recover function is used to continue decoding in the presence of invalid or unrepresentable sequences. This includes both those detected by encode returning InvalidSequence and those that occur because the input byte sequence appears to be truncated.

Progress will usually be made by skipping the first element of the from buffer. This function should only be called if you are certain that you wish to do this skipping and if the to buffer has at least one element of free space. Because this function deals with decoding failure, it assumes that the from buffer has at least one element.

recover may raise an exception rather than skipping anything.

Currently, some implementations of recover may mutate the input buffer. In particular, this feature is used to implement transliteration.


close :: IO ()

Resources associated with the encoding may now be released. The encode function may not be called again after calling close.

getState :: IO state

Return the current state of the codec.

Many codecs are not stateful, and in these case the state can be represented as '()'. Other codecs maintain a state. For example, UTF-16 recognises a BOM (byte-order-mark) character at the beginning of the input, and remembers thereafter whether to use big-endian or little-endian mode. In this case, the state of the codec would include two pieces of information: whether we are at the beginning of the stream (the BOM only occurs at the beginning), and if not, whether to use the big or little-endian encoding.

setState :: state -> IO ()

data TextEncoding Source

A TextEncoding is a specification of a conversion scheme between sequences of bytes and sequences of Unicode characters.

For example, UTF-8 is an encoding of Unicode characters into a sequence of bytes. The TextEncoding for UTF-8 is utf8.


forall dstate estate . TextEncoding 


textEncodingName :: String

a string that can be passed to mkTextEncoding to create an equivalent TextEncoding.

mkTextDecoder :: IO (TextDecoder dstate)

Creates a means of decoding bytes into characters: the result must not be shared between several byte sequences or simultaneously across threads

mkTextEncoder :: IO (TextEncoder estate)

Creates a means of encode characters into bytes: the result must not be shared between several character sequences or simultaneously across threads


type CodeBuffer from to = Buffer from -> Buffer to -> IO (CodingProgress, Buffer from, Buffer to) Source

data CodingProgress Source




Stopped because the input contains insufficient available elements, or all of the input sequence has been sucessfully translated.


Stopped because the output contains insufficient free elements


Stopped because there are sufficient free elements in the output to output at least one encoded ASCII character, but the input contains an invalid or unrepresentable sequence